Traditional total hip replacements involve inserting a stem of a femoral implant into the medullary canal of a patient's femur after the femur has been resected at the distal end of the femoral neck. The stem is usually tapered such that its sides gradually converge from a wider proximal end to a narrower distal end. This configuration allows the stem to fill the majority of the medullary canal as the femur gradually narrows in a distal direction and this helps to anchor the implant in the femur. A rounded tip is provided at the distal end of the stem and a femoral neck and head is provided at the proximal end. The femoral head is constituted by a spherical ball configured for location within a corresponding acetabular cup.
A femoral stem implant may be designed for cemented or uncemented use. In relation to uncemented stems there are two principal designs. The most common design comprises a flat tapered stem with parallel anterior and posterior surfaces. Previously, tubular stems were common but now tapered tubular stems are more common. All known designs have a porous proximal coating for bone in-growth and all work reasonably well.
Nearly all known stem designs have a medial curvature to match the medial curvature of the inside of the femur. However, nearly all designs ignore the curvature of the inside of the lateral wall of the femur and have a straight or angled lateral border instead.
US2012/0172996 discloses a stem design that includes a medial to lateral inward taper in a proximal section and a lateral to medial inward taper in a distal section. Accordingly, the taper direction changes along the length of the stem such that there are relatively large load bearing areas on the wide medial surface proximally and on the wide lateral surface distally. It is claimed that this helps to promote self-centering of the stem in the medullary canal so as to avoid varus or valgus mal-alignment. Notably, the stem is design to prevent rotation of the implant during and after insertion.
It is an aim of the present invention to provide an improved femoral stem prosthesis.